The present invention generally relates to the field of laser optics and, more specifically, to an apparatus for varying the length of an etalon so as to select a desired transmission frequency.
An etalon is a resonant optical cavity defined by two highly parallel substantially reflective surfaces. Etalons are commonly used to obtain narrowed spectral outputs from a gas laser. In such applications, etalons are placed within the optical cavity of the gas laser and manipulated, as described below, to select a desired frequency or frequency range output for the laser. The selection of the desired spectral output of a laser is referred to in the art as "tuning." The use of etalons and the technique of "tuning" a gas laser using an etalon are well known in the art.
Typically, an etalon placed in the resonant optical cavity of a gas laser is positioned such that the reflective parallel surfaces of the etalon are misaligned (i.e., tilted) with respect to the laser's cavity mirrors. This misalignment optically decouple the etalon from the resonant cavity and, consequently, enables the etalon to act as a bandpass transmission filter. The advantage of using a tilted etalon is that the characteristics of the bandpass filter are largely unaffected by small motions of the etalon with respect to resonant optical cavity of the laser. The technique of using a tilted etalon to tune a gas laser is described in detail in Hercher, "Tunable Single Mode Operations of Gas Lasers Using Intracavity Tilted Etalons," Applied Optics, June 1969, Vol. 8, No. 6 at pages 1103 through 1106, which is incorporated herein by reference.
In operation, an etalon of length d and refractive index n, when tilted an angle, R, with respect to the incident light beam will shift the resonant wavelength, g, of the incident beam in the following manner: EQU vg--gR.sup.2 /2n.sup.2
expressed in terms of frequency, f, the shift may be expressed: EQU vf=fR.sup.2 /2n.sup.2
Hence, the tuning curve of a tilted etalon (i.e., resonant wavelength or frequency versus angle of tilt) depends only on the refractive index of the etalon and is independent of the length of the etalon.
The frequency selectivity depends both on the etalon length, d, and the reflectivities of its surfaces. The transmission, T, of the etalon may be approximated by: EQU [1+(2F/p).sup.2 sin.sup.2 ((2pnd cos(R/n))/g))].sup.-1,
where F is the finesse of the etalon, and where the absorption and scattering losses are negligible compared with the transmission at each surface. This also assumes the two reflecting surfaces are highly parallel so that after multiple reflections the light beam retraces essentially the same path. In the best case, an etalon placed inside the resonant optical cavity of a laser can select one of the discrete axial mode frequencies of the laser cavity. Because the presence of the tilted etalon has an effect upon the optical length, L, of the resonant optical cavity, the resonant frequency, f.sub.c, of the resonant optical cavity is likewise shifted.
As discussed in greater detail in the Hercher article cited above, one very typical use of a tilted etalon in the gas laser environment is in the single frequency operation of an argon ion laser. If, through the use of an intracavity dispersing prism, a cw argon laser is operated on a single transition, the spectral output will generally be a rapidly fluctuating function of time. An intracavity tilted etalon can be used in order to obtain a stable single frequency output from such a laser. In such an application, for example, the free spectral range of the etalon (i.e., c/2nd) should equal or exceed the gain bandwidth of the laser.
As can be plainly seen from the preceding discussion, the ability to select particular frequencies using a tilted etalon depends largely on the ability to manipulate controlling parameters. The simplest and most common parameter manipulated is the etalon length d. Indeed, by manipulating the etalon length d by one-half wavelength of the radiation, the desired transmission frequencies or frequency ranges may be tuned.
Two types of systems for manipulating etalon length are most common in prior art systems.
The first type of system varies the etalon optical length using temperature tuning. The entire etalon is normally fabricated of a single piece of fused quartz. This material has a coefficient of optical index of refraction change with temperature that is many times its length change coefficient. It is heated or cooled through a small temperature range to tune the etalon.
This system is disadvantageous because a significant amount of time is expended in heating or cooling the body of the etalon in order to change frequencies. Another disadvantage of this type of system is the excessive cost involved in manufacturing etalons and heating equipment of sufficiently acceptable tolerances for use in the optical laser environment.
A third problem with this "solid" etalon occurs at high power levels, where small light absorption in the coatings and the fused quartz causes very significant heating effects within the glass which detunes the etalon.
The second type of system uses mechanical compression to vary the length of the etalon. In such compression-based systems, the etalon is hollow and is placed under a load such that the length of the etalon varies as a function of the load applied to the etalon. The load in such systems is typically provided by a helical compression spring which acts in mechanical cooperation with an adjustment knob or screw. The knob or screw is tightened or loosened depending upon the amount of the load to be applied to the etalon. The spring applies the load directly upon the etalon.
Use of a helical compression spring is advantageous from a cost perspective as springs, and the associated hardware used to compress the etalon, are inexpensive and relatively easy to manufacture. Moreover, tuning of the laser is virtually instantaneous. The heating and cooling periods required by thermal expansion systems is eliminated so as to enable rapid tuning. An additional advantage of the mechanically tuned etalon is that the body can be made of very low expansion materials, so that no temperature control is necessary.
The disadvantage of these prior art compression-based systems resides in the physical nature of the helical compression spring used to apply the load. Although the annular face of the spring which contacts the etalon may be milled relatively flat, the distribution of the load, regardless of the precision of the involved milling, will not be continuous about the circumference of the annular face due to the inherent physical characteristics of the spring. The load can vary, in some cases, approximately twenty-five percent about the circumference of the annular face of the compression spring.
Because the load varies about the circumference of the annular face of the spring, the load will not be applied uniformly about the circumference of the etalon. As a consequence of this uneven distribution of the load acting on the etalon, the etalon will have one of its parallel surfaces misaligned with respect to the other. As a result of misalignment, the transmission efficiency of the etalon and its ability to precisely tune desired frequencies (i.e., "bandwidth") are materially impaired.
To date, there has been no way to uniformly distribute the load applied to an etalon so as to enable highly reliable and accurate compression tuning.